Overheat problem at idle 2007 Mercury 115 HP EFI 4 Stroke

[WY1953]

One day my outboard engine just started to alarm while idling toward the dock and no water coming out of the tell tail at idle. The smart craft gauge said it was engine temp. I thought no big deal I would replace the water pump impeller. I purchased a new impeller and removed the lower unit and removed the water pump housing. The old impeller didn't look that bad, but I replaced it any way. I checked the water inlet it looked clean. Put it back together and back in the water. Same thing would happen , no water out the tell tail at idle and I could watch the temp gauge rise. If I throttle it up above 1000 rpm it will start spurting water out the tell tail and the temp gauge will go back to about 145 degrees. I checked the thermostat and it is working fine. Pulled the fitting out of the engine that the fuel cooler and the tell tail hook into, the screen on it is clean. The engine runs fine at all rpms except the lack of water at idle rpm. I noticed that there is a lot of water coming up around the driveshaft in the front ,below the engine at higher rpms. I am wondering if there may be a hole in the pump discharge tube or something like that. I have the factory service manual for this engine, but it does not have a flow chart of the water cooling system. I do not see any water flow control valve in the manual either. Has anyone had a similar problem with this series engine? Does anyone have a water flow chart for this engine? I bought the boat used with about 700 hours on it and has 780 on it now. As far as I know it has always been ran in fresh water. The engine S/N is 1B3818**. I have searched the internet and haven't found much about this engine.

 

[Texasmark]

If your engine is like the 2 strokers in cooling methodology, and no reason why it wouldn't be similar where similarities can exist, there is a thermostat (142F opening temp on my last 90 hp) for speeds below 2500 rpms and a bypass "poppet" for higher speeds where more flow is needed and temp regulation isn't necessary.

On some engines the tell tale is plumbed off the thermostat has to be open to get any water out. On engines directly plumbed off the exhaust manifold (on 2 strokers, maybe yours too) water is available as soon as the water pump gets water pumped up to it.

On squirting water out other places on the engine/lower unit, only other place would be the exhaust relief hole(s) on the rear of the engine below the cowl at the top of the mid section. Water/water vapor/nothing comes out of there depending on engine and operating conditions.

On some engines the "pitot tube" used for water pressure for your speedometer operation is sourced at the leading edge of the lower unit (just a hole in it) roughly an inch or so below the antivent plate. A hose is connected to that outlet that is intended to go to your speedometer and if not connected will squirt water when underway in F gear.

 

[WY1953]

I have been over the service manual several times and there is no reference to a poppet or water control valve. The speedometer tube is connected and works fine. The water is coming up from the mid shaft housing when in neutral and throttling up to say 2000 rpm and above, while at the dock. So not just when the boat is moving. I have read that some mercury 4 strokes have a corrosion problem on the upper end of the water tube where the grommet seal enters the adapter plate. Maybe this is what is going on with mine. I ordered a complete new water pump and will replace mine. I will look at the things a little closer when I have it apart.